1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, for diagnosing an abnormality of a fuel supply system from a air/fuel ratio feedback correction or the like, or for improving the air/fuel ratio control under high temperature in the engine compartment.
2. Description of Related Art
Regarding a control apparatus for an internal combustion engine, when a fuel supply system component, such as a fuel injection valve or a fuel pump, is troubled or deteriorated, the basic air/fuel ratio (i.e., a basic fuel injection amount) highly deviates from a target value, and the air/fuel ratio feedback correction amount becomes excessively high. In this respect, Japanese Patent Application Laid-Open No. 4-318250 discloses a fuel control apparatus, in which the abnormality of the fuel supply system is diagnosed by comparing the air/fuel ratio feedback correction amount with a predetermined abnormality determination value. Considering that the fluctuation of the basic air/fuel ratio (and the fluctuation of the air/fuel ratio feedback correction amount) becomes vigorous during a transient drive, moreover, the disclosure prohibits the diagnosis of the abnormality of the fuel supply system at the transient drive so as to prevent the diagnosis failure caused by the fluctuation of the air/fuel ratio during the transient drive.
However, driving condition with a possibility that the basic air/fuel ratio highly deviates while the fuel supply system is normally working, corresponds not only to the transient drive but also to the situation in which the temperature in the engine compartment is high. The reasons for this may be as follows:
(1) When the engine compartment temperature is high, the fuel temperature in the fuel pipe of the engine compartment also becomes hot to produce vapor (or bubble) in the fuel. As a result, the fuel to be injected from the fuel injection valve contains the vapor so that the fuel injection amount is decreased and the air/fuel ratio is shifted to a leaner side. Especially in a system having a returnless pipe system (the system in which the return pipe, for returning to the fuel tank an excessive fuel in a delivery pipe for distributing the fuel to the fuel injection valves of the individual cylinders, is omitted), the fuel in the fuel pipe of the engine compartment has no circulation. Therefore, the fuel temperature in the fuel pipe is liable to rise in the fuel pipe, and is liable to enlarge the deviation of the fuel injection amount due to the fuel vapor; PA1 (2) When the temperature in the engine compartment is high, the sensors (e.g., an air flow sensor, an intake pipe pressure sensor or an intake air temperature sensor) of the air/fuel ratio control system and the fuel injection valves turn hot, so that the characteristics of these parts are chaged. PA1 (3) When the temperature in the engine compartment is high, the intake air temperature rises in the course of the intake air flowing through the intake air pipe to the engine, so that the difference between the intake air temperature measured by the intake air temperature sensor and the actual engine intake air temperature becomes large to cause a deviation in the intake air temperature correction coefficient of the air/fuel ratio.
Since those causes (1) to (3) may enlarge the deviation from the basic air/fuel ratio when the engine compartment temperature is high, it is conceivable to prevent the diagnosis failure by prohibiting the abnormality diagnosis for the fuel supply system at the high temperature in the engine compartment as well as the transient drive.
However, since the high temperature in the engine compartment often continues longer than the transient drive, the prohibition of the abnormality diagnosis while the engine compartment temperature is high may elongate the diagnosis prohibition period too much. This raises a problem that the discovery of an abnormality may be delayed.
Furthermore, if the internal combustion engine is run under a high load for a long time, the temperature of the engine may still be hot even after it is stopped, and vapor (or evaporated fuel gas) may be liable to be generated in the fuel pipe. When the engine is restarted under the situation that the engine is still hot (referred to as the "hot restarting"), the fuel injection rate is made lower than the demanded value by the vapor, so that the air/fuel ratio shifts to a lean side. This lowers the accuracy of the learned value of the air/fuel ratio, so that a proper air/fuel ratio feedback control cannot be achieved. For this solution, the air/fuel ratio learning is prohibited during the hot restarting in an Examined Published Japanese Patent Application No. 7-26579 (JP-B2-7-26579).
During the prohibition of the air/fuel ratio learning, the deviation of the air/fuel ratio due to the vapor has to be absorbed exclusively by the feedback correction coefficient. As illustrated in FIGS. 14A to 14E, therefore, there is a tendency that the feedback correction coefficient during the air/fuel ratio learning prohibition becomes large. When the air/fuel ratio feedback control is then stopped by cutting the fuel or the like, the feedback correction coefficient is reset to an initial value (1.0). When the temporarily stopped air/fuel ratio feedback control is restarted, therefore, the feedback correction is made to allow the feedback correction coefficient to restore the initial value (1.0) to a proper value. However, since the difference between the initial value and the proper value of the feedback correction coefficient is large during the air/fuel ratio learning prohibition, it takes a long time to restore the feedback correction coefficient to the proper value by the air/fuel ratio feedback control. Thus, there arises a problem that the deviation P arises in the air/fuel ratio for the long time period thereby to affect the exhaust emission or the like adversely.